Influenza is a human disease that leads every year to >30,000 deaths in the US and several hundred thousand deaths globally (1). Major neutralization antigenic proteins, hemagglutinin (HA) and neuraminidase (NA) on the virion surface, provide protecting immunity, but undergo yearly genetic variation by point mutations (genetic drift). This renders the viruses resistant to population immunity and set the stage for seasonal epidemics. Further, influenza virus may acquire a new antigenic make-up (reassortment of heterologous genes, referred to as genetic shift) leading to pandemics. Because the flu is seasonal and variable, new vaccines must be produced every year. This is made more complex since more than one type or strain of influenza virus co-circulates in any flu season, a phenomenon demanding that more than one new vaccine may have to be developed every year.
Currently, only two major types of vaccines are licensed, the intramuscularly administered inactivated vaccines (“Flu shot”), and the live attenuated vaccine (LAIV), given intra-nasally (“FluMist®”). The efficacy of the two vaccines is suboptimal. The injectable inactivated vaccines that requires a large quantity of starting material (the equivalent of approximately 1010 plaque-forming units, PFU, per dose), are incapable of inducing significant cell-mediated immunity, which is being recognized as an important determinant of protection (4). Moreover, the overall efficacy of the inactivated vaccine in the U.S. adult population aged 18-65 years is only 59% (5). The LAIV “FluMist,” on the other hand, induces both humoral and cellular immunity but it is restricted in use to people 2 to 49 yr of age (6, 7). Moreover, recurrent administration of LAIV, which always uses the same attenuating viral backbone, could result in tolerance in repeat recipients (8).
Influenza viruses that have been classified as type A, B, and C, are enveloped, negative-strand RNA viruses of Orthomyxoviridae of which subtypes of type A are the major culprit of human disease (3). The viruses transcribe and replicate their multipartite genome in the cell nucleus, each segment encoding one or two polypeptides. Of these the most important antigenic molecules are the glycoproteins hemagglutinin (HA) and neuraminidase (NA).